Return value after searching 3 col's of zipcodes

Hi everyone, I know the basics but this is beyond me.

I run a small courier Co and charge a fixed rate between towns in set zones, I have 3 columns of Zip-codes (Postcodes here in Oz) corresponding to the zones and wish to input two codes and return a dollar value based on what zone they are in.

My criteria is for example:
Any two codes from zone one is $20.00
Any two codes from zone two is $22.50
Any two codes from zone three is $25.00
Any one code from zone one and either of the other zones is the $ value of the higher zone plus $8.00 and the same for between zones two and three (the higher value plus $8.00)

I hope this makes sense and is do-able.

Oscar